Recently, a digital broadcast system has been proposed which uses MPEG2 (Moving Picture Experts Group Phase 2) to compress and encode video data and audio data and then broadcasts encoded streams of data over a ground wave or a satellite wave. This digital broadcast system generates a plurality of transport packets for a plurality of programs each consisting of encoded video streams and encoded audio streams by dividing elementary data such as video data or audio data of each program into a predetermined number of bytes and appending a header to the beginning of each block of divided data. Then these transport packets are multiplexed to broadcast them over a ground wave or a satellite wave.
When a reception device receives transmitted data, that is, multiplexed transport packets, it can obtain the encoded video streams and encoded audio streams of each program by reading out the header information of each transport packets from the received data and restoring the original unmultiplexed elementary data based on the header information.
In such a digital broadcast system, a program generally consists of a plurality of data elements (video data and audio data for a plurality of channels). Therefore, for such a digital broadcast system, it is desirable that an audience should subscribe for each data element contained in a program. For example, suppose that a program consists of a total of four types of data elements, that is, video data, main audio data, subaudio data, and additional data. For a prior digital broadcast system, an audience had to subscribe for each program, that is, all of six data elements. Accordingly, even if a recipient wanted to subscribe only for video data and main audio data, he or she had to subscribe for also unnecessary elements (subaudio and additional data).
Moreover, since a prior scramble device was used before multiplexing, the apparatus must include a scramble device for each program and was unavoidably made large.
In addition, a prior digital broadcast system multiplexed program subscription information and cryptanalytic keys into programs at certain intervals and then transmitted them. Since the highest bit rate of multiplexed streams is limited, the transmission rate may not be sufficiently high for a program due to the transmission of such program subscription information and cryptanalytic keys. Therefore, there was a problem that if a sufficient transmission rate could not be ensured, a transmission buffer for buffering any program data might overflow.